Turbot is an important flatfish widely distributed along the European coasts, whose fishery is centered in the North Sea. The commercial value of the species has boosted a successful aquaculture sector in Europe and China. Body growth is the main target of turbot breeding programs and is also a key trait related to local adaptation to temperature and salinity. Differences in growth rate and optimal growth temperature in turbot have been shown to be associated with a hemoglobin polymorphism reported more than 50 years ago. Here, we identified a Gly16Asp amino acid substitution in the β1 globin subunit by searching for genetic variation in the five functional globin genes within the whole annotated turbot genome. We predicted increased stability of the turbot hemoglobin by the replacement of the conserved Gly with the negative charged Asp residue that is consistent with the higher rate of αβ dimer assembly in the human J-Baltimore Gly16β->Asp mutant than in normal HbA. The turbot Hbβ1-Gly16 variant dominated in the northern populations examined, particularly in the Baltic Sea, while the Asp allele showed elevated frequencies in southern populations and was the prevalent variant in the Adriatic Sea. Body weight did not associate with the Hbβ1 genotypes at farming conditions (i.e., high oxygen levels, feeding ad libitum) after analyzing 90 fish with high growth dispersal from nine turbot families. Nevertheless, all data at hand suggest that the turbot hemoglobin polymorphism has an adaptive significance in the variable wild conditions regarding temperature and oxygen availability.